As disclosed in Japanese Patent No. 3,259,190, at least five methods are well known as a method of measuring a distortion (distortion component generated when a mask image is transferred onto a wafer) in the projection optical system of an exposure apparatus.
Of these methods, two methods disclosed in Japanese Patent No. 3,259,190 and Japanese Publication No. 63-38697 are proposed as a distortion measurement method using overlay of a main scale mark and a vernier scale mark.
(1) Method Disclosed in Japanese Patent Publication No. 63-38697
According to the method disclosed in this reference, a main scale mark 2 and vernier scale marks 1 which are formed on a test reticle as shown in FIG. 8A are transferred onto the resist layer of a photosensitive substrate, as shown in FIG. 8B. The misalignment amount of the overlay mark after developing (distance from the barycenter of the main scale mark to that of the vernier scale mark) is measured. In inspection, the vernier scale marks 1 on the entire reticle surface are transferred onto the photosensitive substrate. Exposure is repeated by sequentially moving the photosensitive substrate such that the main scale mark 2 overlaps the previously transferred vernier scale marks 1 at a plurality of points.
The photosensitive substrate is moved by a precision moving stage having a high-precision critical dimension measurement device such as a laser interferometer. The moving amount is uniquely determined in correspondence with the designed intervals between the central point and a plurality of points on the reticle. In exposure to the main scale mark 2, the moving stage is moved by a distance corresponding to the interval. The photosensitive substrate which has already been exposed to the vernier scale mark is exposed to the main scale mark 2. An overlay mark 13 as a result of overlaying the main scale mark and vernier scale marks is formed on the entire exposure region of the developed photosensitive substrate, as shown in FIG. 9. These marks are read visually (via a microscope), obtaining an overlay error amount at the target point. If the moving stage is accurately fed, the measurement value (overlay error amount) corresponds to a distortion amount at the target point.
(2) Method Disclosed in Japanese Patent No. 3,259,190.
The method disclosed in this reference uses a reticle having main scale marks 14a and 15a, and vernier scale marks 14b and 15b which are arranged at predetermined small intervals in two directions perpendicular to each other, as shown in FIGS. 10A and 10B. Distortion difference amounts in the two directions perpendicular to each other are transferred onto overlay marks at respective positions. A distortion is derived from the cumulative sum of misalignment amounts obtained by measurement using a microscope.
More specifically, a pattern on the entire surface of a test reticle is transferred onto a substrate by exposure. The substrate holding stage is moved by Δy in the first direction and Δx in the second direction such that the main scale marks 14a and 15a are moved adjacent to the previously transferred vernier scale marks 14b and 15b. Immediately after movement, an overlay mark as shown in FIGS. 11A and 11B is formed by exposure. The misalignment amounts of the two overlay marks formed in this manner exhibit distortion changes in the respective directions. The change amount is divided by the moving amount, obtaining a distortion inclination. The inclination is multiplied by each mark interval to obtain a cumulative sum. The distortion of the entire exposure region can therefore be calculated.
However, the conventional distortion measurement methods described above suffer from the following problems.
In measurement method (1), the feed error of the moving stage is added to the misalignment amount of the overlay mark, resulting in poor measurement precision. If the feed error of the moving stage varies irregularly, the precision can be increased by calculating the average value by a plurality of measurement operations. However, a plurality of measurement operations take a long time, increasing the inspection cost. In the presence of a regular feed error of the moving stage, the precision cannot be increased by a plurality of measurement operations.
Measurement method (2) executes only feed of the moving stage in two directions perpendicular to each other and at least two exposure operations, and the measurement time is short. However, the misalignment amount of the overlay mark which reflects a distortion change is divided by the distance between a corresponding main scale mark and vernier scale mark, obtaining a distortion inclination amount. The distortion inclination amount is multiplied by the distortion measurement interval, obtaining a change amount from an adjacent measurement point. In general, the distance from an adjacent measurement point is longer than the distance between a corresponding main scale mark and vernier scale mark. A small measurement error upon measuring a misalignment amount by a microscope increases by the distance ratio. The increased error is undesirably regarded as the cumulative sum and contained in the distortion.
In the above situation, demands have arisen for higher-precision distortion measurement.